A transient blocking unit (TBU) is a circuit including two or more normally-on transistors that rapidly and automatically switches to a high-impedance current blocking state in response to an over-current condition. A TBU placed in series with a load will thereby protect that load from the over-current condition. For this reason, TBUs find application as series protective elements for sensitive electrical loads. One early reference describing transient blocking units is U.S. Pat. No. 5,742,463.
In many situations, it is important for TBUs to have high voltage handling capability. Once approach for providing such capability is to simply fabricate the basic TBU circuit with transistors having the required voltage rating. However, this approach is often troublesome in practice, because key TBU parameters such as the TBU threshold current (i.e., the current above which the device will turn off) depend on the device parameters of the TBU transistors. It is often difficult and/or costly to provide tight control of device parameters of high voltage transistors. Thus a TBU including high voltage transistors in such a manner that the device parameters of the high voltage transistors determine TBU parameters such as threshold current will suffer from reduced yield and increased cost.
An alternative approach can be referred to as a “core TBU” approach. In this approach, a relatively low voltage TBU is connected to high-voltage transistors such that the overall circuit acts as a high voltage TBU. The use of a low voltage TBU core to control the high voltage transistors can advantageously simplify TBU design. For example, with the TBU core approach, the TBU threshold current is set by the TBU core and advantageously does not depend on (poorly controlled) device parameters of the high voltage transistors. Some aspects of the TBU core approach are considered in US 2006/0098363.
FIG. 1 shows an example of a TBU core circuit as described in US 2006/0098363. In this example, TBU core 102 includes normally-on transistors Q1, Q2, and Q3, and diodes D1 and D2, connected so as to provide transient blocking functionality. TBU core 102 is connected to normally-on high voltage transistors Q4 and Q5 such that when TBU core 102 switches to its high impedance state, at least one of Q4 and Q5 is thereby switched off. Thus, TBU core 102 sets the current threshold, while Q4 and Q5 provide the high voltage handling capability.